1. Field of the Invention
The present invention relates to a linear compressor, particularly but not solely for use in refrigerators.
2. Summary of the Prior Art
Compressors, in particular refrigerator compressors, are conventionally driven by rotary electric motors. However, even in their most efficient form, there are significant losses associated with the crank system that converts rotary motion to linear reciprocating motion. Alternatively a rotary compressor which does not require a crank can be used but again there are high centripetal loads, leading to significant frictional losses. A linear compressor driven by a linear motor would not have these losses, and can be designed with a bearing load low enough to allow the use of aerostatic gas bearings as disclosed in U.S. Pat. No. 5,525,845, where a laterally compliant connecting rod allows for the low bearing load
A discussion of aerostatic gas bearings is included in “Design of Aerostatic Bearings”, J W Powell, The Machinery Publishing Company Limited, London 1970. However with normal manufacturing tolerances and equipment production of effective gas bearings is difficult.
Conventional compressors are mounted within a hermetically sealed housing which in use acts as a reservoir of refrigerant gas. Refrigerant gas is drawn into the compressor from this reservoir and is exhausted through an exhaust conduit leading from the compressor, through the housing.
Operation of the compressor involves the reciprocation of moving parts leading to vibration of the compressor unit, in all three axis. To reduce the external noise effect of this vibration the compressor is mounted on isolation springs within the sealed housing.
With a linear compressor the piston vibrates relative to the cylinder in only one axis, with consequent reaction forces on whichever part, if either, is fixed. One solution proposed to this problem is to operate a pair of compressors synchronously in a balanced and opposed configuration. However this arrangement would be too complex and costly for use in a commodity item such as a domestic refrigerator. Another proposed solution is the addition of a resonant counterweight to reduce the vibration. However this approach limits the operation of the compressor because the counterweight is a negative feedback device and is limited to the fundamental unbalance force. A further solution is proposed in “Vibration characteristics of small rotary and linear cryogenic coolers for IR systems”, Gully and Hanes, Proceedings of the 6th International Cryocooler Conference, Plymouth, Mass., 1990. This solution involves independently supporting the piston part and the cylinder part of the compressor within the housing so that the “stator acts as a counterweight”. However in implementing this design in a domestic refrigerator there is a problem when the piston mass is low. In such a compressor, as the discharge pressure increases, the force of the compressed gas acts as a spring (the “gas spring”) which increases the running speed as the discharge pressure increases. This is a problem because the “third” vibration mode (where the piston and the cylinder vibrate in phase with each other but out of phase with the compressor shell) is only slightly above the desirable “second” mode (where the shell does not vibrate and the piston and cylinder are out of phase). Thus the shell starts to vibrate intolerably as the “gas spring” starts to operate and effectively raises the “second” mode frequency to, and eventually above, the “third” mode.
It is desirable for many applications for the compressor to be of small size. This reduces the size of all components including the springs and their resonant system. Reducing the size of the compressor requires the compressor to run at higher frequencies. Reduced size in higher frequencies combined to increase the stresses in the spring components. In some linear compressors main springs have been made from pressed spring steel sheet. It has been found that the edges cut in the pressing operation require careful polishing to regain the original strength of the spring steel sheet, and are frequently subject to failure through inadvertent stress concentrations.
A further problem of compressors generally has been the build up of heat, particularly in the vicinity of the compressor cylinder and the cylinder head. The build up of heat is caused by friction between moving components and by heat transferred from the compressed refrigerant. The build up of heat creates significant problems of increased wear and of running conditions and tolerances between parts which vary according to the time period for which the compressor has been running. These effects may be particularly significant for linear compressors which may run for long periods of time and for which close clearances are particularly important, especially when aerostatic gas bearing systems are being used.
Another heating effect is from the irreversible heat loss prior to re-expansion from the refrigerant remaining within the compression space after compression. In linear compressors up to 15% of the compressed refrigerant may not be expelled, compared to up to 5% in crank driven compressors. This source of heat, which is negligible in conventional compressors, is an important source in linear compressors.
One approach to cooling the cylinder and cylinder head of the compressor involves using liquid refrigerant supplied from the condenser in the subsequent refrigeration system. For example in U.S. Pat. No. 2,510,887 liquid refrigerant from the condenser is supplied to a first cooling jacket surrounding the cylinder thence to a second cooling jacket surrounding the compressor head and subsequently is ejected from a venturi device into the discharge line connecting between the cylinder head and the condenser. This is in the context of a standard crank driven compressor. Also in the context of a standard crank driven compressor U.S. Pat. No. 5,694,780 shows a circuit in which liquid refrigerant from the condenser is elevated to a higher pressure by a pump. This liquid refrigerant is pumped into a cooling jacket surrounding the compressor cylinder. The liquid refrigerant is forced from the cooling jacket into the exhaust manifold into the cylinder head of the compressor where it mixes with the compressed refrigerant as the compressed refrigerant exits the compressor. This arrangement has the disadvantage of requiring an additional pump for forcing the liquid refrigerant through the cooling jacket surrounding the cylinder and subsequently into the exhaust manifold against the pressure of the compressed gas.
Many linear compressors have been made inside conventional compressor sheets as they have been intended as a drop in replacement for existing rotary-reciprocating compressors which are a commodity item. To achieve this compact size compressors have been made in which the stator, armature, cylinder and piston are all concentrically located. But conventional compressor dimensions constrain the size of the machinery compartment of a refrigerator and lead to wasted space in the compartment surrounding the compressor.